1. Field of the Invention
The present invention relates generally to a technique for image processing that allows detection of modification.
2. Description of the Related Art
Conventionally, techniques with use of special paper or ink have been employed to prevent unauthorized copying or forgery. Developments in printing technology and information and communications technology have allowed easy creation and distribution of documents in recent years. Under such a circumstance, it is typical to produce a printout with a general printer on ordinary paper, while it is less often to perform printing on special paper or with special ink. However, with increasing information security awareness, demands for a technique for preventing leakage of information from and data modification to a document printed on ordinary paper with such a general printer are growing. Not only a technique for detecting modification made to digital data but also a technique for detecting modification made to a printed document has been demanded. Under such circumstances, techniques for preventing modification made to a document printed on ordinary paper have been proposed.
In view of the above circumstances, techniques have been devised. According to, for instance, a technique disclosed in Japanese Patent Application Laid-open No. 2007-5935 or Japanese Patent No. 3964684, second-code image data, which is data for use in detecting modification, is combined with a document image so that modification made to a printed document can be detected. According to another technique disclosed in, for instance, Japanese Patent Application Laid-open No. 2005-72777, two types of image data, or, specifically, background dot patterns and first-code image data, which is additional data, are combined with a document image.
However, the technique disclosed in Japanese Patent Application Laid-open No. 2007-5935 or Japanese Patent No. 3964684 is disadvantageous in that although the second-code image data, which is the data for use in detecting modification, is combined with the document image, the first-code image data serving as the additional data is not combined with the document image.
The technique disclosed in Japanese Patent Application Laid-open No. 2005-72777 is disadvantageous in that although the two types of image data, which are the background dot patterns and the first-code image data serving as the additional data, are combined with the document image, the background dot patterns are not the second-code image data; therefore, modification made to the document image remains undetectable.